Method and apparatus for providing variable positive airway pressure

ABSTRACT

A method and apparatus for treating a breathing disorder and, more particularly, a method and apparatus for providing a pressurized air flow to an airway of a patient to treat congestive heart failure in combination with Cheyne-Stokes respiration and/or sleep apnea or other breathing disorders. A positive airway pressure ventilator is utilized in combination with an algorithm that adjusts IPAP and EPAP in order to counter a Cheyne-Stokes breathing pattern. Cheyne-Stokes respiration is detected by monitoring a peak flow of the patient.

BACKGROUND OF THE INVENTION

[0001] 1. Field Of The Invention

[0002] The present invention relates generally to a method and apparatusfor providing a positive pressure therapy particularly suited treat apatient suffering from congestive heart failure, and, more particularly,to a method and apparatus for providing a pressurized flow of breathinggas to an airway of a patient to treat Cheyne-Stokes respiration, sleepapnea, or other breathing disorders commonly associated with congestiveheart failure.

[0003] 2. Description of the Related Art

[0004] Congestive heart failure (CHF) patients commonly suffer fromrespiratory disorders, such as obstructive sleep apnea (OSA). Anothersuch respiratory disorder CHF patients often experience during sleep isknown as Cheyne-Stokes respiration. FIG. 1 illustrates a typicalCheyne-Stokes respiration (CSR) pattern 30, which is characterized byrhythmic waxing periods 32 and waning periods 34 of respiration, withregularly recurring periods of high respiratory drive (hyperpnea) 36 andlow respiratory drive (hypopnea or apnea) 38. A typical Cheyne-Stokescycle, generally indicated at 40 in FIG. 1, lasts about one minute andis characterized by a crescendo (arrow A), in which the peak respiratoryflow of the patient increases over several breath cycles, anddecrescendo (arrow B) variation in peak flow, in which the peakrespiratory flow of the patient decreases over several breath cycles.The disruption in sleep, as well as the periodic desaturation ofarterial oxygen (PaO₂), stresses the cardio-vascular system andspecifically the heart. Hyperpnea often causes arousals and, thus,degrades sleep quality.

[0005] Relatively recent developments in the treatment of sleep apneaincludes the use of continuous positive airway pressure (CPAP), which isthe application of a constant pressure to the airway of a patient. Thistype of positive airway pressure therapy has been applied not only tothe treatment of breathing disorders, but also to the treatment of CHF.In using CPAP on a CHF patient, the effect of the CPAP is to raise thepressure in the chest cavity surrounding the heart, which allows cardiacoutput to increase.

[0006] Bi-level positive airway pressure therapy is a form of positiveairway pressure therapy that has been advanced in the treatment of sleepapnea and other breathing and cardiac disorders. In a bi-level pressuresupport therapy, pressure is applied to the airway of a patientalternately at relatively higher and lower pressure levels so that thetherapeutic pressure is alternately administered at a larger and smallermagnitude force. The higher and lower magnitude positive prescriptionpressure levels are known as IPAP (inspiratory positive airway pressure)and EPAP (expiratory positive airway pressure), and are synchronizedwith the patient's inspiratory cycle and expiratory cycle, respectively.

[0007] A publication entitled “Effects of Continuous Positive AirwayPressure on Cardiovascular Outcomes in Heart Failure Patients With andWithout Cheyne-Stokes Respiration,” by Don D. Sin et al., which waspublished on Jul. 4, 2000 in Circulation, Vol. 102, pp. 61-66, describeshow CPAP improves cardiac output in patients suffering from CHF andhaving both CSR and central sleep apnea (CSA), which is a cessation ofbreathing for a period of time not due to an obstruction of the airway.Additionally, it was found that CPAP can reduce the combinedmortality-cardiac transplantation rate in patients with combined CSR-CSAwho comply with CPAP therapy.

[0008] One approach to providing a pressure support therapy for thetreatment of cardiac failure, CSR, or CSA is described in InternationalPatent Application Publication No. WO 99/61088 to Resmed Limited (“the'088 publication”). According to the teachings of the '088 publication,a patient is provided with a ventilatory or pressure support using ablower and mask in much the same way as a conventional bi-level pressuresupport system. However, the system also derives an instantaneousventilation of the patient by measuring the volume inspired, the volumeexpired, or half an average volume of the respiratory airflow over ashort period of time. This derived measure of instantaneous ventilationis then used to adjust the level of ventilatory support in an attempt toreduce or eliminate short term changes in the derived measure ofinstantaneous ventilation. This is accomplished by comparing the derivedmeasure of instantaneous ventilation with a target ventilation, which isa relatively long term measure, and controlling the level of pressuresupport based on the error between the two.

[0009] There are disadvantages associated with this method of providingpressure support to a patient to treat cardiac failure, CSR, or CSA. Forexample, in many situations, the average value of the past tidal volumesof the patient will not produce a target ventilation that, in turn, willresult in sufficient treatment of the hypopneas and hyperpneas tocounteract the occurrence of CSR. This is believed to be true becauseCSR has a continuum of severity and, depending on the level of severity,the target ventilation needs to be adjusted to values other than theaverage of the last 1-2 minutes. Moreover, the CHF patient may have somedegree of obstruction that must be treated for its own sake, but alsobecause these obstructive events appear to drive the CSR pattern aswell. Therefore, a system that sets a target ventilation based on along-term average of the past volumes does not address the interplay ofobstructing airways and CSR. Using the instantaneous volume as thefeedback variable requires yet another calculation, and this calculationis prone to errors due to small errors in the estimated patient flow anddetecting the onset and termination of inspiration.

[0010] It is, therefore, desirable to provide a method and apparatus fortreating OSA and CSR commonly found in the CHF population that adjuststhe inspiratory and expiratory pressures to resolve the CSR and OSAbased on detecting the peak flow where the effect of the error in theestimated patient flow is always smaller than that in the subsequentvolume calculation. It is further desirable to detect the presence andseverity of CSR and the level of pressure support presently interveningto treat the CSR more effectively than possible using conventionaltechniques.

SUMMARY OF THE INVENTION

[0011] Accordingly, the present invention provides a method andapparatus for treating sleep apnea and CSR often found in CHF patientsthat does not suffer from the disadvantages associated with presentpressure support treatment techniques. Specifically, the presentinvention implements many of the standard functions of a positive airwaypressure support device, as well as an algorithm that adjusts IPAP,EPAP, or both in order to counter a CSR pattern. The pressure supportsystem includes a pressure generating system and a patient circuitcoupled to the pressure generating system. The pressure generatingsystem includes a pressure generating and a pressure controller, such asa valve, to control the flow of breathing gas from the pressuregenerator. The pressure support system includes a flow sensor to measurethe flow of breathing gas in the patient circuit, and a controller toimplement the algorithm. The output of the flow sensor is used todetermine the peak flow during the patient's respiratory cycles. Thedetected peak flows are monitored to determine whether the patient isexperiencing Cheyne-Stokes breathing.

[0012] Determining and delivering the appropriate IPAP and EPAP is athree layer process each with its own time frame. The first process isexecuted typically 100 times a second and utilizes the aforementionedpressure support system that synchronizes delivery of IPAP and EPAP withthe patient's inspiratory and expiratory drive, respectively. Inaddition to the ventilatory functions, the first process also monitorspeak flow and time capture. The second process is executed every breathcycle, which is typically 10-30 times a minute, and determines the IPAPsetting for the next inspiratory effort based on the previous peak flowand a target peak flow. The third process is executed every 2 to 5minutes and computes indices of CSR shape and severity and thepersistence of the level of pressure support. The CSR shape index is ameasure of how much the last 2-3 minutes of peak flows resembles atypical CSR pattern. The CSR severity index is a ratio of the minimumpeak flow over the maximum peak flow during the last 2 to 3 minutes. Thepressure support persistence is the percentage of breaths that receiveda clinically significant level of pressure support, typically 2 cmH₂O orgreater, over 2 CSR cycles. Based on the CSR shape and severity indicesand the pressure support persistence, the third process will adjusteither the target peak flow, the EPAP or both.

[0013] The level of pressure support is adjusted based on the differencebetween a target peak flow and the last peak flow times a gain.Increases and decreases to the pressure support are limited to typically3 cmH₂O in order to prevent arousals.

[0014] The values for the target peak flow and the timed back up ratecan be selected either by manual control or automatically by compilingstatistics on the flow waveform and measuring the level and persistenceof pressure support that the unit is delivering.

[0015] These and other objects, features and characteristics of thepresent invention, as well as the methods of operation and functions ofthe related elements of structure and the combination of parts andeconomies of manufacture, will become more apparent upon considerationof the following description and the appended claims with reference tothe accompanying drawings, all of which form a part of thisspecification, wherein like reference numerals designate correspondingparts in the various figures. It is to be expressly understood, however,that the drawings are for the purpose of illustration and descriptiononly and are not intended as a definition of the limits of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 illustrates a typical Cheyne-Stokes respiratory cycle thatis treated by the pressure support system of the present invention;

[0017]FIG. 2 is a functional block diagram of a variable positive airwaypressure support system according to the principles of the presentinvention;

[0018]FIG. 3 is a schematic diagram of the process implemented by thepressure support system of FIG. 2 to treat Cheyne-Stokes respiration;

[0019]FIGS. 4A, 4B and 4C illustrate pressure waveforms delivered by thepressure support system of FIG. 2 according to the principles of thepresent invention;

[0020]FIG. 5A is a chart illustrating the patient flow for a normalpatient and FIG. 5B is a chart illustrating the patient flow for apatient experiencing CSR;

[0021]FIG. 6A is a chart showing an array of peak flow data collected bythe variable positive airway pressure support system, FIG. 6B isillustrates the array of peak flow data after a first DC bias removalprocess, and FIG. 6C illustrated the array of peak flow data normalizedfor comparison to an exemplary a CSR template waveform used by thesystem to gauge the effectiveness of the pressure support treatment;

[0022]FIG. 7 is an exemplary embodiment of a look-up table used toadjust the target peak flow in the process shown in FIG. 3;

[0023]FIG. 8 is an exemplary embodiment of a look-up table used toadjust the EPAP level in the process shown in FIG. 3.

[0024]FIG. 9 is a chart illustrating the patient flow for a patientsuffering from CSR and the pressure waveform output by the variablepositive airway pressure support system of the present invention inresponse thereto;

[0025]FIG. 10 is a pressure waveform illustrating exemplary changes toboth IPAP and EPAP that can be accomplished by the variable positiveairway pressure support system of the present invention; and

[0026]FIG. 11 is a diagram illustrating a CSR monitoring processaccording to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

[0027] There is generally indicated at 50 in FIG. 2 a variable positiveairway pressure support system according to a presently preferredembodiment of the instant invention and shown in the form of afunctional block diagram. Pressure support system 50 is operable toimplement a novel mode of pressure support, referred to herein as avariable positive airway pressure (VarPAP) mode, for delivering a flowof breathing gas for the treatment of Cheyne-Stokes respiration.

[0028] Variable positive airway pressure support system 50 includes agas flow generator 52, such as a blower used in a conventional CPAP orbi-level pressure support device, that receives breathing gas, generallyindicated by arrow C, from any suitable source, e.g., a pressurized tankof oxygen or air, the ambient atmosphere, or a combination thereof. Gasflow generator 52 generates a flow of breathing gas, such as air,oxygen, or a mixture thereof, for delivery to an airway of a patient 54at relatively higher and lower pressures, i.e., generally equal to orabove ambient atmospheric pressure. The pressurized flow of breathinggas, generally indicated by arrow D from gas flow generator 52 isdelivered, via a delivery conduit 56, to a breathing mask or patientinterface 58 of any known construction, which is typically worn by orotherwise attached to a patient 54 to communicate the flow of breathinggas to the airway of the patient.

[0029] Delivery conduit 56 and patient interface device 58 are typicallycollectively referred to as a patient circuit.

[0030] Variable positive airway pressure support system is illustratedin FIG. 2 as a single-limb system, meaning that the patient circuitincludes only a delivery conduit 56 connecting the patient to thepressure support device. As such, an exhaust vent 57 is provided in thedelivery conduit for venting exhaled gasses from the system as indicatedby arrow E. It should be noted that the exhaust vent can be provided atother locations in addition to or instead of in the delivery conduit,such as in the patient interface device. It should also be understoodthat the exhaust vent can have a wide variety of configurationsdepending on the desired manner in which gas is to be vented from thepressure support system.

[0031] The present invention also contemplates that the variablepositive airway pressure support system can be a two-limb system, havinga delivery conduit and an exhaust conduit connected to the patient. In atwo-limb system, the exhaust conduit carries exhaust gas from thepatient and include an exhaust valve at the end distal from the patient.The exhaust valve is typically actively controlled to maintain a desiredlevel or pressure in the system, which is commonly known as positive endexpiratory pressure (PEEP).

[0032] In the illustrated exemplary embodiment of the present invention,patient interface 58 is a nasal mask. It is to be understood, however,that patient interface 58 can include a nasal/oral mask, nasal pillows,tracheal tube, endotracheal tube, or any other device that provides thegas flow communicating function. Also, for purposes of the presentinvention, the phrase “patient interface” can include delivery conduit56 and any other structures that connects the source of pressurizedbreathing gas to the patient.

[0033] In the illustrated embodiment, variable positive airway pressuresupport system 50 includes a pressure controller in the form of a valve60 provided in delivery conduit 56. Valve 60 controls the pressure ofthe flow of breathing gas from flow generator 52 delivered to thepatient. For present purposes, flow generator 52 and valve 60 arecollectively referred to a pressure generating system because they actin concert to control the pressure and/or flow of gas delivered to thepatient.

[0034] It should be apparent that other techniques for controlling thepressure delivered to the patient by the pressure generator, such asvarying the blower speed, either alone or in combination with a pressurecontrol valve, are contemplated by the present invention. Thus, valve 60is optional depending on the technique used to control the pressure ofthe flow of breathing gas delivered to the patient. If valve 60 iseliminated, the pressure generating system corresponds to pressuregenerator 52 alone, and the pressure of gas in the patient circuit iscontrolled, for example, by controlling the motor speed of the pressuregenerator.

[0035] Variable positive airway pressure support system 50 furtherincludes a flow sensor 62 that measures the flow of breathing gas withindelivery conduit 56. In accordance with a presently preferred embodimentshown in FIG. 2, flow sensor 62 is interposed in line with deliveryconduit 56, most preferably downstream of valve 60. Flow sensor 62generates a flow signal Q_(measured) that is provided to a controller 64and is used by the controller, as discussed below, to determine the flowof gas at the patient Q_(patient). Of course, other techniques formeasuring the respiratory flow of the patient are contemplated by thepresent invention, such as measuring the flow directly at the patient orat other locations along delivery conduit 54, measuring patient flowbased on the operation of the pressure generator, and measuring patientflow using a flow sensor upstream of the pressure generator.

[0036] An input/output device 66 is provided for setting variousparameters used by the variable positive airway pressure support system,as well as for displaying and outputting information and data to a user,such as a clinician or caregiver. It is to be understood that thepresent invention contemplates providing input/output terminals so thatthe operation information and data collected by the variable positiveairway pressure support system can be monitored and controlled remotely.Controller 64 is preferably a microprocessor that is capable ofimplementing and executing routines for monitoring characteristics ofpatient respiration and controlling the flow of breathing gas basedthereon as discussed in detail below.

[0037] The details of variable positive airway pressure support system50 and its operation are discussed below with reference to FIGS. 3-10.FIG. 3 schematically illustrates the process carried out by controller64 in implementing the VarPAP mode of pressure support. Controller 64,in general, performs three sets of processes to manage or control thepressure support delivered to the patient when operating in the VarPAPmode.

[0038] These three sets of operations are differentiated from oneanother by lines 68 and 70 in FIG. 3.

[0039] The first set of processes, which are below line 70, are carriedout essentially continuously by the controller in accordance with theclock speed of the processing unit in controller 64, such as at a rateof 100 Hz. The second set of processes, which are between lines 68 and70, are carried out less frequently, such as during every respiratorycycle or every other respiratory cycle, typically 10-30 times perminute. They can be executed, for example, at each trigger point, wherethe pressure support system transitions from EPAP to IPAP, i.e., at thetransition from expiration to inspiration, or at each cycle point, wherethe pressure support system transitions from IPAP to EPAP, i.e., at thetransition from inspiration to expiration. The third set of processes,which are above line 68, are carried out even less frequently, such asevery 1-5 minutes. In a preferred embodiment of the present invention,the operation above line 68 are carried out every 2 minutes. While thefirst, second and third sets of operations are described above in thepreferred embodiment of the present invention as being executed at arate of 100 Hz, every breath, and every 2 minutes, respectively. It isto be understood, however, that other rates can be used, so long as thefunction or functions of each set of operations is sufficientlyachieved.

[0040] The basic operations of the pressure support system in providinga pressure support therapy to a patient are accomplished by pressuresupport process block 72 in FIG. 3. In a preferred embodiment of thepresent invention, the variable positive airway pressure support systemessentially functions as a bi-level pressure support system, and,therefore, includes all of the capabilities necessary in such systems inorder to provide separate IPAP and EPAP levels to the patient. Thisincludes receiving the necessary parameters via input commands, signals,instructions or information 74 for providing a bi-level pressure, suchas maximum and minimum IPAP and EPAP settings. The flow signalQ_(measured) from flow sensor 62 is also provided to the pressuresupport process, which controls the pressure controller to output thedesired inspiratory and expiratory waveforms. Typically, carrying putthe pressure support operation includes estimating or determining theactual patient flow Q_(patient) based on the flow signal Q_(measured),determining whether the patient is in the inspiratory or expiratoryphase of the respiratory cycle and providing an I/E state signalindicative of the respiratory state of the patient, and triggering andcycling the pressure support system. The outputs to the pressurecontroller are not illustrated in FIG. 3 for ease of illustration.

[0041] In a preferred embodiment of the present invention, which is asingle-limb system, controller 64 in process block 72 estimates theleakage of gas from the pressure support system using any conventionaltechnique and incorporates this leak estimation into the determinationof the actual patient flow Q_(patient). This leak estimation is requiredin a single-limb system, because a single-limb system includes a knownleak through the exhaust vent as well as other unknown leaks, such asleaks at the patient contact site of the patient interface and atvarious conduit couplings on the patient circuit. In a two-limb system,leak estimation may not be required, because a flow sensor is typicallyprovided at the exhaust vent to measure, directly, the flow of exhaustgas. In such a system, the patient flow Q_(patient) can be determined bysubtracting the measured exhaust flow from the measured flow deliveredto the patient. It can be appreciated that leak detection can beperformed in a two-limb system to increase the accuracy of the patientflow determination.

[0042] U.S. Pat. No. 5,148,802 to Sanders et al., U.S. Pat. No.5,313,937 to Zdrojkowski et al., U.S. Pat. No. 5,433,193 to Sanders etal., U.S. Pat. No. 5,632,269 to Zdrojkowski et al., U.S. Pat. No.5,803,065 to Zdrojkowski et al., and U.S. Pat. No. 6,029,664 toZdrojkowski et al., as well as pending U.S. patent appln. Ser. No.09/586,054 to Frank et al., the contents of each of which areincorporated by reference into the present invention, describe how toaccomplish the necessary functions in order to provide separate IPAP andEPAP levels to the patient, which are the functions accomplished inprocess block 27. These functions include techniques for detecting andestimating leak, and techniques for detecting the respiratory state of apatient, and managing, e.g., triggering and cycling, the bi-leveldelivery of breathing gas to the patient in the presence of leaks. Thus,a detailed discussion of these functions is omitted from the presentapplication for the sake of simplicity and brevity.

[0043] In a preferred embodiment of the present invention, controller64, in process block 72, controls pressure generator 52, pressurecontroller 60, or both to deliver a VarPAP waveform 76, as generallyshown in FIG. 4A, to an airway of patient 54. VarPAP waveform 76 isessentially a bi-level pressure waveform that alternates between an IPAPlevel and an EPAP level. According to the present invention, the IPAPand EPAP levels are variable under the controller of controller 64 asdiscussed below. Therefore, maximum and minimum IPAP and EPAP levels(IPAP_(max), IPAP_(min), EPAP_(max) EPAP_(min)) are provided to thecontroller as inputs 74 to process block 72. It should be understoodthat the maximum and minimum IPAP and EPAP levels can also bepreestablished and stored in the controller as a default or in lieu ofinput parameters from the system operator. In a preferred embodiment ofthe present invention, the minimum IPAP level is set to a level that issufficient to treat OSA.

[0044] As shown in FIG. 4A, at time F, which is the trigger point fromexpiration to inspiration, the patient begins inspiring and triggers thepressure support system to transition to an IPAP level 80. The shape andduration of the pressure increase or rise 82 from trigger point F to theIPAP level can be fixed or variable, as taught for example, in U.S. Pat.Nos. 5,598,838 and 5,927,274 both to Servidio et al. and copending U.S.Patent Application No. 60/216,999, to Yurko, the contents of each ofwhich are incorporated herein by reference. It should be understood thatthe present invention contemplates that the inspiratory portion ofpressure waveform 76 can have a variety of configurations.

[0045] At time G, at the end of the inspiratory period, which is thecycle point from inspiration to expiration, the patient begins theexpiratory phase of the breathing cycle and the pressure support systemcycles, causing the pressure to drop toward an EPAP level, indicated at84. In the illustrated embodiment, the waveform for expiratory pressure,P_(exh), output by the pressure support system during the expiratoryphase of the breathing cycle is determined according to the followingequation:

P _(exh) =EPAP+Gain_(exh)* Flow,  (1)

[0046] where Gainexh is a gain factor, typically selected by acaregiver, and Flow is the estimated patient flow Q_(patient). U.S. Pat.Nos. 5,535,738; 5,794,615; and 6,105,575 all to Estes et al., thecontents of which are incorporated herein by reference, teach thistechnique for controlling the expiratory pressure delivered by abi-level pressure support system. As a result, the pressure delivered tothe patient drops below EPAP at area H during patient exhalation,thereby increasing patient comfort. In FIG. 3, an expiratory pressurerelief process 86 receives the patient flow Q_(patient) and implementsequation (1) for generating the expiratory pressure waveform P_(exh),which is then supplied to process block 72.

[0047] It is to be understood that the present invention contemplatesthat the expiratory portion P_(exh) of pressure waveform 76 can have avariety of configurations. For example, FIG. 4B illustrates a VarPAPwaveform 88 in which the expiratory pressure waveform P_(exh)corresponds to the expiratory pressure administered by a conventionalbi-level pressure support system, wherein the EPAP level remaingenerally constant throughout the expiratory phase of the breathingcycle. This is accomplished, for example, by eliminating expiratorypressure relief process 86 in FIG. 3. In addition, FIG. 4C illustrates aVarPAP waveform 90 in which the inspiratory portion and the expiratoryportions of the waveform are at the same pressure level, so thatIPAP=EPAP. Thus, VarPAP waveform 90 effectively corresponds to aconventional CPAP waveform.

[0048] Referring back to FIG. 3, the patient flow Q_(patient) is alsoprovided to a peak flow detection process 92, which monitors the patientflow and identifies the peak flows Q_(peak(current)) 94 occurring withinthat flow during the inspiratory portion of each respiratory cycle. Seealso FIGS. 5A and 5B. The level of the peak Q_(peak(current)) during theinspiratory phase of each respiratory cycle is stored in a memory arrayin peak flow, time and PS storage process 96 in FIG. 3. In addition tostoring the peak flow for each breath, a time stamp identifying when thepeak flow occurred, and an indication of the level of pressure supportbeing provided to the patient at that time are also stored in the memoryarray. The pressure support level (PS_(level(current))) is determined asthe difference between IPAP and EPAP. In other words,PS_(level)=IPAP−EPAP. As discussed below, this stored information isused in other process blocks to determine how well the pressure supportsystem is functioning to treat CSR and to adjust the system parameters,if necessary.

[0049] As noted above, a characteristic of CSR is the presence of ahypopnea or apnea period 38 between the hyperpnea periods 36. SeeFIG. 1. These periods are often referred to as central apneas, becausethe cessation of respiration during these intervals is not believed tobe due to an occluded airway. The VarPAP pressure support mode of thepresent invention addresses these periods of apnea in an automaticbackup process 98. In automatic backup process 98, an I/E state signal,which indicates the respiratory state of the patient and, hence,identifies transitions between the states, is monitored. If a centralapnea or cessation of respiratory effort is detected for a period oftime, for example 5-12 seconds and referred to as the apnea detectiontime T_(apnea), then a “machine breath” is automatically delivered tothe patient by the pressure support system, thus ventilating the lungs.In the embodiment shown in FIGS. 5A and 5B, the apnea detection timeT_(apnea) begins at the end of each inspiration. If a period of timeT_(apnea) passes before the next spontaneous inspiration, a “machinebreath” is automatically delivered to the patient. Such an event isgenerally indicated at 99 in FIG. 5B, where the T_(apnea) period beganat point I. Because a spontaneous inspiration had not occurred after theelapse of this period, a machine generated backup breath 101 wasinitiated at point J.

[0050] The apnea detection time T_(apnea) is set manually orautomatically based on that patient's prior breath rate, such as anaverage of the expiratory periods of the patient's last n breaths, wheren is an integer. Once a machine breath is delivered to the patient, thepressure support system continues to deliver backup breaths at a back-upbreath rate T_(breath), with each machine generated “breath” having aninspiratory time T_(insp). Typically, the backup breath rate T_(breath)and the inspiratory time T_(insp) are set by a clinician and areprovided by the automatic backup process to the pressure supportprocess, as indicated by signals 100, to control the operation of thepressure support system so that the system provides the machinegenerated breaths to the patient. Backup breaths cease when the patientbegins breathing spontaneously, which is detected in pressure supportprocess 72 and which appears as a change in the I/E state signalprovided to automatic backup process 98.

[0051] Referring again to FIG. 3, according to the VarPAP algorithm ofthe present invention, the IPAP level is adjusted based on the resultsof a comparison between the last measured peak flow Q_(peak(current))and a target peak flow Q_(peak(target)) in an IPAP adjustment process102. The last measured peak flow Q_(peak(current)) is received from peakflow detection process 92 and the target peak flow Q_(peak(target)) isprovided by a target peak flow and EPAP adjustment process 104 discussedin greater detail below. In a preferred embodiment of IPAP adjustmentprocess 102, a change in the IPAP level (ΔIPAP) is calculated duringeach respiratory cycle using the following equation:

ΔIPAP=Gain(Q_(peak(target))−Q_(peak(current))),  (2)

[0052] where Gain is a fix gain determined empirically. In a preferredembodiment of the present invention, the Gain is selected as 9 cmH₂O. Itis to be understood, that this gain can be varied, as needed, manually,or can be automatically adjusted over a prescribed range by the IPAPadjustment process.

[0053] Once the ΔIPAP for a breath is calculated, the new IPAP level(IPAP_(new)) for the next following breathing is determined asIPAP_(new)=IPAP_(previous)+ΔIPAP. Preferably, the rate of change for theIPAP level, i.e., the ΔIPAP, is limited so that the patient is notpresented with an abrupt change (increase or decrease) in the IPAPlevel. In a preferred embodiment of the present invention, ΔIPAP islimited to ±3 cm H₂O. The new IPAP level is also checked against theestablished IPAP_(max), and IPAP_(min) levels. Note that IPAP_(min)should not be less than the current EPAP level. The new IPAP level isthen used by pressure support process 72 in the next inspiratory cycle.In this manner, the VarPAP pressure support algorithm continuously, ateach respiratory cycle, searches for the appropriate EPAP level to bedelivered to the patient based on the measured peak flow and a targetpeak flow.

[0054] It is to be understood that other techniques for adjusting theIPAP level in IPAP adjustment process 102 are contemplated by thepresent invention. For example, the current peak flow Q_(peak(current))can be compared to the target peak flow Q_(peak(target)), and ifQ_(peak(current)) is less than Q_(peak(target)), the IPAP level isincreased. Likewise, if Q_(peak(current)) is greater thanQ_(peak(target)), the IPAP level is decreased. Moreover, the amount bywhich the IPAP level is increased, i.e., the ΔIPAP, can be variabledepending on the amount by which Q_(peak(current)) differs fromQ_(peak(target)). That is, the greater the difference, the greater theΔIPAP.

[0055] Ideally, the IPAP pressure control function of IPAP adjustmentprocess 102 and the operation of automatic backup process 98 to treatthe central apnea phase of the CSR cycle are sufficient to counteractthe CSR. However, this may not be the case for all patients. Inaddition, the condition of each patient is dynamic. For these reasons,among others, the present invention includes the ability to adjustautomatically the degree to which the pressure support system attemptsto counteract the CSR cycle. This is accomplished by providing, intarget peak flow and EPAP adjustment process 104, the ability to alterthe target peak flow Q_(peak(target)) used in IPAP adjustment process102 automatically. The EPAP level can also be altered automatically,either alone or in combination with a target peak flow adjustment, bytarget peak flow and EPAP adjustment process 104 to treat the occurrenceof CSR more effectively than in a system that uses a static EPAP level.The decision whether to adjust the target peak flow Q_(peak(target)),the EPAP level, or both, and the amount to which they are adjusted isdetermined in target peak flow and EPAP adjustment process 104 based onthe results of a performance parameter determination process 106.

[0056] In essence, in performance parameter determination process 106the VarPAP pressure support mode assess the degree to which the patientis experiencing CSR, if at all, and based on this determination, thevariable positive airway pressure support system adjusts the pressuresupport provided to the patient in target peak flow and EPAP adjustmentprocess 104 by adjusting the target peak flow, the EPAP, level or both.As noted above, this process of measuring the effectiveness of theperformance of the VarPAP mode of pressure support and process ofadjusting the pressure support to increase its effectiveness, ifnecessary, is carried out every 2 to 5 minutes.

[0057] In a preferred embodiment of the present invention, performanceparameter determination process 106 measures of the effectiveness of thepressure support therapy and determines the degree of pressure supportintervention based on the following three parameters: 1) a CSR shapeindex, 2) a CSR severity index, and 3) a pressure support (PS) index.Each of these parameters is discussed in turn below. Based on the CSRshape index, the CSR severity index, PS index, the target peak flow andEPAP adjustment process will adjust either the target peak flow, theEPAP or both.

[0058] The CSR shape index is determined based on a coherence function,which is a mathematical tool for determining how well an unknown patternis similar to a template pattern. In the present invention, the unknownpattern is a sequence of previously recorded peak flows, and thetemplate pattern is a pattern selected to correspond to a CSR pattern.The CSR shape index, expressed as a percentage, is a measure of how wellthese two patterns coincide, and, hence, how well the peak flow datacollected over the past several minutes corresponds to a CSR pattern;the closer the match, the more likely it is that the patient isexperiencing CSR.

[0059] The coherence technique first requires acquiring the stored peakflows and associated data from peak flow, time and PS storage process 96corresponding to the peak flows stored over the last 2-3 minutes. Thepeak flows are then processed to fit a typical CSR pattern of a leastone cycle, approximately 60 sec. in duration. Depending on the CSRtemplate, this requires that peak flows and times from the last 2-5minutes to be stored in the array in peak flow, time and PS storageprocess 96. Using a normalized cross-correlation technique, the peakflows are compared to the CSR template, and a CSR shape index rangingfrom 0-100% is generated.

[0060]FIG. 6A illustrates an array of peak flows 108 (Q_(peak)(i))stored in peak flow, time and PS storage process 96 over the timeinterval of interest, which is typically the last 2-5 minutes. The peakflows 108 are processed to remove the “DC” bias in this array of peakflow flows, so that zero crossings 112 can be detected to yield ashifted array of peak flows 110 (Q_(peak)′(i)) shown in FIG. 6B.According to an exemplary embodiment of the present invention, the “DC”bias is removed by first searching the array of peak flow for minimumand maximum peak flow values Q_(peak(min)) and Q_(peak(max)), and thenrecalculating each peak flow Q_(peak)′(i) according to the followingequation:

Q _(peak)′(i)=Q _(peak)(i)−(Q _(peak(max)) −Q _(peak(min)))/2+Q_(peak(min)),  (3)

[0061] where i is the sample index. Of course, any conventionaltechnique for effectively removing the DC bias, i.e., placing a zeroline in the peak flow array Q_(peak)(i) 108 at the appropriate locationcan be used, so that it is then possible to determine the zero crossings112 of the shifted array of peak flows Q′_(peak)(i) 110.

[0062] To find the zero crossings, the shifted array of peak flowsQ′_(peak)(i) 110 is searched, preferably starting at the most recentQ_(peak)′(i) and working backwards in time, using a robust zero crossing(ZC) detection method. The first three zero crossings 112 having thesame slopes are used to define the last two CSR cycles 114. Once a ZC isdetected, it is also time-stamped. From the ZC time-stamps, the periodT_(CSR) of the CSR cycle is measured. The measured CSR periods are usedto time-wrap each of the two CSR cycles on to the CSR template.Excessive time-warp due to the measured CSR period being out of range,e.g., 40-90 seconds, stops the process, and a CSR Index of 0% (zero) isreturned.

[0063] The CSR template is a sequence of peak flows that describe thegeneral shape of CSR. In a preferred embodiment of the present inventionshown in FIG. 6C, simple triangle function is used as CSR template 116.It can be appreciated, however, that other templates can be used.

[0064] To time-warp the Q_(peak)′(i) array, the time stamps and theQ_(peak)′(i) values are used to map Q_(peak)′(i) values on to the samesampling rate as the CSR template 116 using linear interpolation and,thus, a second array of peak flows Q_(peak)″(i) 118 is produced as shownin FIG. 6C. To perform the correlation in the discrete-time domain,i.e., using digital samples, the samples in the peak flow array have tobe time-aligned with those of CSR template 116. The coherence function,which provides an indication of the degree of difference betweenQ_(peak)″(i) and the CSR template is computed. The result is called theCSR shape index, which is given in percent and ranges from 0 to 100%.

[0065] In summary, the peak values are stored in an array along with thetimestamps of when the peaks occurred. Next, the first threezero-crossings are detected and the periods of the first two CSR cyclesare computed. The peak flow array is recalculated and time-warped inorder to fit the CSR template and the coherence function is computedyielding the CSR shape index.

[0066] The CSR severity index is calculated from the array of peak flowsQ_(peak)(i) 108 (FIG. 6A) as a ratio of the minimum peak flowQ_(peak(min)), over the maximum peak flow, Q_(peak(max)). The lastminimum and maximum values or an average of several minimum and maximumvalues occurring in the array of peak flows during the sample intervalcan be used to determine the CSR severity index, which is also expressedas a percentage. In general, a CSR severity index greater than 50% isconsidered normal, less than 50% is abnormal and an index of 0%indicates the occurrence of a central apnea.

[0067] The pressure support (PS) index, unlike the CSR shape index andthe CSR severity index, is not a measure of a parameter directlyassociated with the CSR cycle. Rather, the PS index is a measure ofamount of assistance that is being provided by the pressure supportsystem in attempting to combat the CSR cycle, i.e., how much thepressure support system is intervening on behalf of the patient toaugment their ventilation. The amount of pressure support is determinedas the difference between the IPAP and the EPAP levels of the pressureprovided to the patient, i.e., PS=IPAP−EPAP. In a preferred embodiment,the PS index is determined as the percentage of breaths where thepressure support provided to the patient was above a threshold,typically 2 cmH₂O over the last 2-3 minutes or last 2 CSR cycles.

[0068] The CSR shape index, a CSR severity index, and PS indexdetermined in performance parameter determination process 106, areprovided to target peak flow and EPAP adjustment process 104, whichdetermines whether to adjust the target peak flow Q_(peak(target)), theEPAP level, or both based thereon. More specifically, in a target peakflow adjustment mode, the target peak flow and EPAP adjustment processuses the table shown in FIG. 7 to adjust the target peak flow based onthe ranges for the CSR shape index, CSR severity index, and PS index.The EPAP level is not adjusted. The extreme right column 113 in FIG. 7indicates the change ΔQ_(peak(target)) to be made at each adjustmentdepending on the ranges for the CSR shape index, CSR severity index, andPS index.

[0069] It should be noted that the present invention is not intended tobe limited to the values shown for the change in the peak flow targetΔQ_(peak(target)), i.e., the values shown in column 113 in FIG. 7. Onthe contrary, those skilled in the art can appreciated that the changein the peak flow target can be modified according to clinical judgment.

[0070] If, however, the PS index is 70% or more for more than 6 minutes,the target peak flow is not adjusted. Instead, the target peak flow andEPAP adjustment process enters an EPAP adjustment mode. It is to beunderstood that the present invention is not intended to be specificallylimited to the 70% and 6 minute selections for transitioning to the EPAPadjustment mode. On the contrary, those skilled in the art wouldunderstand that a range of index valves and time limits are possible.

[0071] In the EPAP adjustment mode, target peak flow and EPAP adjustmentprocess 104 uses the table shown in FIG. 8 to adjust the EPAP levelbased on the ranges for the CSR shape index, CSR severity index, and PSindex determined by performance parameter determination process 106. Theextreme right column 115 in FIG. 8 indicates the change ΔEPAP to be madeat each adjustment. This EPAP change, or the new EPAP setting, isprovided to pressure support process 72 as indicated by blocks 120 a and120 b in FIG. 3. In this mode, the target peak flow Q_(peak(target)) isdecreased incrementally during each EPAP adjustment process by apredetermined amount, such as 1 liter per minute, until the PS index isless than 50%, for example.

[0072] It should be noted that the present invention is not intended tobe limited to the values shown for the change in the EPAP level, i.e.,the values shown in column 115 in FIG. 8. On the contrary, those skilledin the art can appreciated that the change in the EPAP can be modifiedaccording to clinical judgment.

[0073] As noted above, preferably maximum and minimum target peak flowQ_(peak(target(max))), Q_(peak(target(min))) and maximum and minimumEPAP levels are set so that the target peak flow and the EPAP levels arelimited to a range of permissible, clinically safe, values.

[0074] The EPAP adjustment continues until 1) the CSR is resolved, asdetermined by performance parameter determination process 106, or 2) theEPAP level reaches its maximum for 6 minutes. Of course, other EPAPlevels and time limits for determining when to end the EPAP adjustmentmode can be selected depending on the desired degree at which the systemattempts to correct the CSR pattern. When either of these conditionsoccur, the EPAP level is then decreased toward its minimum level, andthe target peak flow and EPAP adjustment process 104 switches back tothe target peak flow adjustment process, where EPAP remains constant andthe target peak flow is adjusted according to the table shown in FIG. 7.

[0075] In an example of an application of the variable positive airwaypressure support system, the preferred embodiment of the presentinvention is connected to a patient as shown in FIG. 2. A caregiver,such as an authorized clinician, doctor, or respiratory therapist,inputs a minimum and maximum IPAP and EPAP levels, as well as a minimumand maximum target peak flow Q_(peak(target(max))),Q_(peak(target(min))). Initially, the IPAP level is set to the minimumIPAP pressure, EPAP is set to the minimum EPAP pressure, and the targetpeak flow is set to the minimum target peak flow setting. If the minimumIPAP and EPAP levels are the same, the variable positive airway pressuresupport system operating in this VarPAP modes is essentially providing aCPAP pressure support treatment at that time, because IPAP=EPAP. Thepatient then falls asleep with the device running the VarPAP algorithmshown in FIG. 3. If the peak flow Q_(peak) drops below the target peakflow value Q_(peak(target)), IPAP is increased by an incremental amountfor the next breath.

[0076]FIG. 9 illustrates the patient flow Q_(patient) 122 for a patientsuffering from CSR and a pressure waveform 124 output by the variablepositive airway pressure support system of the present invention inresponse thereto. In time interval 126, the patient is experiencing adecrescendo in his or her respiratory peak flows. During this interval,the IPAP level is at a minimum level and the EPAP level 128 is constant.However, at the start of time interval 130, the patient's peak flowfalls below the target peak flow and IPAP level 132 in increased andcontinues to increase, as indicated by increasing IPAP peaks 134, 136,and 138 during interval 130. At a time, generally indicated at 140, thepatient does not make an inspiratory effort, and a machine breath 142 isdelivered by the pressure support system. In time interval 144, thepatient receives additional machine generated breaths until a time 146,when the patient makes a spontaneous respiratory effort. During thesubsequent time interval 148, the IPAP level is reduced back to itsminimum.

[0077] Every 2 to 5 minutes, the patient flow is monitored usingperformance parameter determination process 106 to determine whether thepatient's flow corresponds to a CSR pattern. If so, the target peakflow, EPAP, or both are altered as discussed above. FIG. 10 illustratesa pressure waveform 153 output by the pressure support system thatincludes changes to both IPAP and EPAP, according to the principles ofthe present invention. In this illustration, during time interval 150,the patient is receiving repeated patterns of pressure support where theEPAP level 152 is constant and the IPAP level 154 repeatedly increases,as generally indicated at 156, and decreases, as generally indicated at158. During these intervals, the pressure support system determines, viaperformance parameter determination process 106 and target peak flow andEPAP adjustment process 104, that the patient is experiencing CSR and iscontinues to attempt to counteract the CSR by adjusting the target peakflow.

[0078] At the beginning of time interval 160, however, the system beginsincreasing the EPAP level, because altering the peak target flow was notsufficient to counteract the presence of CSR. In other words, the systemhas switched to the EPAP adjustment mode discussed above. Duringinterval 60, the EPAP level increases, as generally indicated at 162,while the IPAP level also continues to be adjusted by IPAP adjustmentprocess 102. Eventually, either the CSR pattern is resolved, or theEPAP_(max) level is reached. In either event, the EPAP level isdecreased, as generally indicated at 164, during interval 166. Duringthis interval, the peak target pressure is also decreased. Thereafter,the pressure support system exits the EPAP adjustment mode and returnsto the peak target pressure Q_(peak(target)) adjustment mode.

[0079] As noted above, during the apnea period of the CSR cycle, amachine breath in delivered to the patient. When this occurs, specialconsideration must be given to the peak flows determined by peak flowdetection process 92 in FIG. 3. If, for example, the peak flowsresulting from the machine generated breaths are used for the patient'speak flow, this may not be an accurate representation of the patient'sactual respiratory condition, because the machine generated peak flowsare not indicative of the patient's respiratory effort. For this reason,when a machine breath is administered, the pressure support system setsthe peak for Q_(peak) to zero, i.e., Q_(peak(current))=0, even thoughthe measured peak flow Q_(patient) corresponds to the peak flow producedfrom the machine generated breath. This prevents the peak flow arrayQ_(peak)(i) (see FIG. 6A) from inaccurately resembling a normal patientflow pattern, especially during the central apnea portion of the CSRcycle. For later machine generated breaths, if any, the peak flow is notset to zero. Instead, the peak flow Q_(peak(current)) is set to themeasured peak flow minus a correction factor, which is a function of thepeak flow in the second machine generated breath.

[0080] More specifically, the peak flow that corresponds to the secondmachine generated breath Q_(peak(t)0) is determined according to thefollowing equation:

Q _(peak)(t0)=Q _(peak)(t0)−G _(rs) *PS(t0),  (4)

[0081] where PS(t0) is the pressure support at time t0(PS(t0)=IPAP(t0)−EPAP(t0)), and G_(rs) is determined as follows:$\begin{matrix}{G_{rs} = {\frac{Q_{peak}({t0})}{{PS}({t0})}.}} & (5)\end{matrix}$

[0082] The peak flows that corresponds to the third, fourth, etc.,machine generated breaths Q_(peak)(t), which are, of course, after thesecond machine generated breath Q_(peak)(t0), are determined accordingto the following equation:

Q _(peak)(t)=Q _(peak)(t)−G _(rs) *PS(t).  (6)

[0083] It can be appreciated that the above-described techniques forimplementing the VarPAP mode of pressure support controls the pressuresupport levels for IPAP and EPAP based on how closely a series ofpatient's peak flows correspond to a series of peak flows associatedwith CSR. This is determined, in particular, in performance parameterdetermination process 106 as discussed above. However, the presentinvention contemplates other techniques for determining whether thepatient is actually experiencing CSR. An alternative CSR detectionalgorithm 168 to detect CSR, and, hence, control the target peak flow,EPAP, or both based thereon, is shown in FIG. 11.

[0084] In this embodiment, the process shown in FIG. 11 is executedcontinuously to “look for” the CSR cycle. According to this technique,three consecutive increasing peak flows are considered an upward trendand likewise, three consecutive peak flows decreasing in value areconsidered a downward trend. This alternative algorithm detects theupward trend (crescendo), peak (hyperpnea), downward trend(decrescendo), and valley (hypopnea or apnea) of the CSR pattern. If theupward or downward trends are broken then the present apparatus resets.Otherwise, if the present apparatus completes two cycles, then thesystem determines that a CSR pattern is detected, and adjusts the targetpeak pressure or the EPAP level accordingly. For example, the targetpeak pressure is first adjusted to its maximum if CSR is detected. Ifthis fails to counteract the CSR cycle, the EPAP level is then increasedand the target peak pressure is lowered. All the while, IPAP adjustmentprocess 102 continues to optimize the IPAP setting.

[0085] CSR detection in FIG. 11 begins in step 170, where the currentpeak flow Q_(peak)(k), which is the peak flow for the current breathcycle, is compared to the previous peak flow Q_(peak)(k−1), which is thepeak flow for the immediately preceding breath cycle. If Q_(peak)(k) isgreater than Qpeak(k−1), the process attempts to determine if there is aupward trend (crescendo) in step 172. If three consecutive reductions inpeak flow are present, the process looks for a peak flow peak(hyperpnea) in step 174. Otherwise, the process returns to step 170. Ifa peak is detected in step 174, the process attempts to determine ifthere is a downward trend (decrescendo) in step 176. If threeconsecutive decreases in peak flow are present in step 176, the processlooks for a peak flow valley (hypopnea or apnea) in step 178. If a peakin the peak flow is detected in step 178, the process begins lookingagain for three consecutive reductions in peak flow in step 172. If thecycle is completed two or more times, the patient is deemed to beexperiencing CSR. A similar process is followed if Q_(peak)(k) is lessthan Q_(peak)(k−1) in step 170.

[0086] Some breathing patterns exhibit the CSR pattern but have minorfluctuations in peak flow. A true CSR pattern shows high peak flowfollowed by a very low peak flow or an apnea. Thus, in a preferredembodiment of the present invention, a further criteria, in which themaximum peak flow has to be above the threshold, must be met before thepatient is considered to be experiencing that hyperpnea phase of a CSRpattern. Likewise, a still further criteria, in which the minimum peakflow has to be below the hypopnea level, must be met before the patientis considered to be experiencing that hyponea phase of a CSR pattern.These thresholds are determined by the clinician and typically based onobservation of peak flows during sleep.

[0087] In a further embodiment of the present invention, the pressuresupport system is also adapted to implement other conventional modes ofpressure support, such as CPAP, PPAP, BiPAP, for delivering the flow ofbreathing gas to treat sleep apnea, including obstructive sleep apneaand central apneas, CHF, COPD, or other cardio-pulmonary disorders,either alone or in conjunction with the novel VarPAP pressure supportmode for treating CSR of the present invention.

[0088] The present invention contemplates that controller 26 implementsmany of the standard functions of a pressure support device, i.e.,providing CPAP, bi-level pressure support BiPAP, PPAP pressure support,smart-CPAP as taught, for example, in U.S. Pat. Nos. 5,203,343;5,458,137; and 6,087,747 all to Axe et al. the contents of which areincorporated herein by reference, or auto-CPAP as taught, for example,in U.S. Pat. No. 5,645,053 to Remmers et al. the contents of which arealso incorporated herein by reference, in addition to implementing theVarPAP mode of pressure support. In one embodiment of the presentinvention, the pressure support system includes a mode select inputdevice that allows a user or authorized caregiver to select the mode ofventilation (VarPAP, CPAP, bi-level, auto-CPAP) under which the pressuresupport device operates. However, the present invention alsocontemplates that pressure support system implements the VarPAP mode ofpressure support alone. In addition, the present invention contemplatesperforming the CSR detection techniques in the background whileimplementing a conventional mode of pressure support and then switchingthe VarPAP mode of pressure support once CSR is detected.

[0089] If not otherwise stated herein, it may be assumed that allcomponents and/or processes described heretofore may, if appropriate, beconsidered to be interchangeable with similar components and/orprocesses disclosed elsewhere in the specification, unless an indicationis made to the contrary.

[0090] It should be appreciated that the apparatus and methods of thepresent invention may be configured and conducted as appropriate for theapplication. The embodiments described above are to be considered in allrespects only as illustrative and not restrictive. The scope of theinvention is defined by the following claims rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

1-18. (Cancelled).
 19. A system for delivering a flow of gas to anairway of a patient, the system comprising: a gas flow generating systemthat generates a flow of gas; a patient circuit coupled to the gas flowgenerating system and adapted to communicate the flow of gas to anairway of a patient; a sensor associated with the gas flow generatingsystem or the patient circuit and adapted to measure a characteristicassociated with the flow of gas within the patient circuit and totransmit a flow signal indicative thereof; target value determiningmeans for monitoring an effectiveness of the system in treatingCheyne-Stokes respiration of such a patient and for setting a targetvalue based on the monitored effectiveness; CSR monitoring means fordetermining whether such a patient's respiratory activity is indicativeof Cheyne-Stokes respiration by comparing the characteristic associatedwith the flow of gas with the target value; and controlling means forcontrolling the gas flow generating system so as to cause the gas to bedelivered at a sufficient pressure during at least a portion of abreathing cycle to treat Cheyne-Stokes respiration based on an output ofthe CRS monitoring means.
 20. The system of claim 19, wherein the gasflow generating system includes: a pressure generator adapted togenerate the flow of gas; and a pressure control valve associated withthe pressure generator or the patient circuit to control a pressure ofthe flow of gas delivered to a patient by the patient circuit.
 21. Thesystem of claim 19, wherein the target monitoring means sets a value fora target peak flow Q_(peak(target)) as the target value based on whethersuch a patient's respiratory activity is indicative of Cheyne-Stokesrespiration, wherein the CSR monitoring means compares an inspiratorypeak flow Q_(peak(current)) to the target peak flow Q_(peak(target)),and wherein the controlling means causes the gas flow generating systemto adjust a pressure of the flow of gas based on the comparison betweenthe inspiratory peak flow Q_(peak(current)) and the target peak flowQ_(peak(target)).
 22. The system of claim 21, wherein the CSR monitoringmeans adjusts the value of the target peak flow Q_(peak(target)), anexpiratory positive airway pressure, or both, responsive to adetermination that the delivering of pressurized gas in the treatment isnot effectively treating Cheyne-Stokes respiration.
 23. The system ofclaim 19, wherein the controlling means detects I/E transitions betweenan inspiratory phase and an expiratory phase of a respiratory cycle, andcauses the gas flow generating system to deliver a machine breathresponsive to a failure to detect a patient initiated breath betweenwithin a predetermined period of time.
 24. The system of claim 19,wherein the CSR monitoring means determines magnitudes and times ofinspiratory peaks flows and comparing these to a Cheyne-Stokesrespiration pattern.
 25. The system of claim 19, wherein the targetvalue determining means, the CSR monitoring means, the controllingmeans, or a combination thereof are implemented on a common processingsystem.
 26. A method for delivering pressurized gas to an airway of apatient, the method comprising the steps of: delivering a flow of gas tothe airway of the patient from a source of gas via a patient circuit;sensing a characteristic associated with a flow of gas within thepatient circuit and outputting a flow signal; monitoring aneffectiveness of the system in treating Cheyne-Stokes respiration ofsuch a patient and setting a target value based on the monitoredeffectiveness; determining whether such a patient's respiratory activityis indicative of Cheyne-Stokes respiration by comparing thecharacteristic associated with the flow of gas with the target value;and controlling the delivery of gas so as to cause the flow of gas to bedelivered at a sufficient pressure during at least a portion of abreathing cycle to treat Cheyne-Stokes respiration based on a result ofthe determining step.
 27. The method of claim 26, wherein delivering aflow of gas includes generating a flow of gas via a pressure generatorand controlling a pressure of the flow of gas via (1) a pressure controlvalve associated with the pressure generator or the patient circuit, (2)controlling an operating speed of the pressure generator, or (3) acombination of both (1) and (2).
 28. The method of claim 26, whereinsensing a characteristic associated with a flow of gas within thepatient circuit includes determining an inspiratory peak flowQ_(peak(current)), wherein monitoring the effectiveness of the systemincludes determining a target peak flow Q_(peak(target)) as the targetvalue, wherein determining whether such a patient's respiratory activityis indicative of Cheyne-Stokes respiration includes comparing theinspiratory peak flow Q_(peak(current)) to the target peak flowQ_(peak(target)), and wherein controlling the delivery of gas includesadjusting a pressure of the flow of gas based on the comparison betweenthe inspiratory peak flow Q_(peak(current)) and the target peak flowQ_(peak(target)).
 29. The method of claim 28, wherein monitoring aneffectiveness of the system in treating Cheyne-Stokes respirationincludes adjusting the target peak flow Q_(peak(target)), an expiratorypositive airway pressure, or both, responsive to a determination thatthe delivering of pressurized gas in the treatment is not effectivelytreating Cheyne-Stokes respiration.
 30. The method of claim 26, furthercomprising: detecting I/E transitions between an inspiratory phase andan expiratory phase of a respiratory cycle; and delivering a machinegenerated breath to a patient responsive to a failure to detect apatient initiated breath between within a predetermined period of time.31. The method of claim 26, wherein monitoring whether such a patient'srespiratory activity is indicative of Cheyne-Stokes respiration includesdetermining magnitudes and times of inspiratory peaks flows andcomparing these to a Cheyne-Stokes respiration pattern.